We will make a phylogenetic survey of invertebrate muscle to determine the distribution of paramyosin, using sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis of myofibrillar or cell homogenates of glycerinated muscles as a method of paramyosin detection. We propose to determine the amount of paramyosin/muscle, relative both to total myofibrillar or cell contractile protein and to myosin heavy chains, using quantitative SDS gel electrophoresis. We then will localize paramyosin within sarcomeres, using immunochemical techniques. Sarcomere lengths will be predetermined by laser diffraction, and paramyosin localization at different sarcomere lengths will be compared with results previously obtained with Limulus striated muscle. Using the results of these experiments and those of the morphological studies of our collaborator, Dr. Maynard M. Dewey, SUNY at Stony Brook, N. Y., we will attempt to determine whether or not a relationship exists between organization of elements of the contractile apparatus and the relative paramyosin content of the different muscle types. We hope to establish a role for paramyosin in those muscles in which it is found.